1. Field of the Invention
The present invention relates to integrated optical devices and, more particularly, to an optical device contemplated for tapping signal power from an optical waveguide whereby the tapped signal is substantially independent of polarization and wavelength.
2. Description of the Prior Art
Due to the increase in the use of optical fiber communication channels, the development of integrated optical devices for directly processing optical signals has become of significant importance to system designers. One particularly useful approach for optical processing is through the use of integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging" 7 J. Lightwave Technol., pp. 1530-1539 (1989). In essence, a silicon substrate is provided with a base layer of SiO.sub.2 and a thin core layer of doped silica glass is deposited on the oxide. The core layer can be configured to a desired waveguide structure-typically 5-7 micrometers wide-using standard photolithographic techniques, and a layer of doped silica glass is deposited on the core to act as a top cladding. Depending on the precise configuration of the waveguide, such devices can perform a wide variety of functions including tapping of signal power from the optical waveguide.
In a typical signal tapping application of the aforedescribed integrated optical devices, two waveguides are passed in close adjacency for a length, i.e., coupler length, dependent upon the desired degree of coupling. Energy from one waveguide core is transferred to an adjacent core to effectuate the signal tap.
One shortcoming of such optical tap configurations is that the tapped signal tends to be dependent upon the wavelength of the signal. Another shortcoming concerns the birefringence induced in the waveguide by the strain of the glass layers. The strain is due to the difference in thermal expansion of the glass films composing the waveguide and the substrate. It is compressive when the waveguides are formed on silicon substrates and it's magnitude varies with layer composition. Such strain-induced birefringence presents different indices of refraction for the different polarization modes i.e., the transverse magnetic (TM) mode and the transverse electric (TE) mode of the transmitted light. The effect of this is that the mode confinement is polarization dependent and, consequently, the coupling of two waveguides becomes polarization dependent. Thus, a tapped signal is provided, which is dependent on the polarization state of the signal.
Several techniques have been suggested for overcoming the intrinsic birefringence of glass-on-silicon waveguides. One method employs a half-wave plate inserted in the middle of a waveguide grating multiplexer to rotate the polarization by 90.degree.. See H. Takahashi, et al., "Polarization-Insensitive Arrayed-Waveguide Multiplexer on Silicon" Opt. Letts. 17(7), p 499 (1992). This approach, however, leads to excessive loss. Another approach is to deposit on the waveguide a layer (six micrometers) of amorphous silicon. A drawback of this approach is that the silicon layer must be then actively trimmed with a high power laser.
Accordingly, there exists a need for further improvements in compensating for wavelength dependencies and strain-induced birefringence in integrated optical tap devices.